Advanced, efficient solar collectors of the type disclosed in U.S. Pat. Nos. 3,952,724; 4,043,318; 4,018,215; and 4,033,327 are currently in use. The collectors are comprised of glass concentric tubes, one within the other, and sealed together to provide an annular space between them that is evacuated to a hard vacuum i.e. 10.sup.-4 Torr. The outer "cover tube" is transparent and the inner "absorber tube" is selectively coated over the surface that is in the vacuum space. These tubes are inserted in liquid tight connections in a manifold and a liquid, e.g. water, is pumped into each of the absorber tubes to fill them and is pumped through the tubes in series to extract the heat of the solar radiation absorbed by the absorber tubes.
The collectors also are equipped with various reflectors, as is set out in U.S. Pat. No. 4,022,160, such that diffuse as well as direct sunlight reach the absorber tubes of the device.
It is most significant to utilize water as the heat extraction medium for many uses and for many reasons; which include the economy and availability of water, its desirable thermal coefficients as a heat extraction medium, and its non-toxic properties. On the other hand, water freezes at relatively undesirable high temperature in field service and during inactive periods in cold climates, such as in the northern regions of the United States in winter, requiring precautions against freeze up in the system. Heretofore, mixes of anti-freeze compounds in the water have been used as one way to combat this problem, however, such measures introduce toxic properties to the heat extraction media.
Also, in the prior systems wherein water is used with or without anti-freeze compounds in mixtures, the collector system must be operated continuously to extract the heat of the solar energy collected so as to prevent excessive operating temperature buildup on the tubes. Accordingly, as an example of domestic hot water demand or heating and cooling demands or the combination thereof in a solar operated installation there are periods, such as in spring or fall seasons, when excess solar energy is being absorbed by the collector of the system over the needs or demand for thermal energy thereby.
The excess collection requires "dumping" some of the energy, that is, the excess must be diverted to special air handlers or cooling towers resulting in nonproductive energy cost and equipment expenditure of a system.
In some systems, such as in U.S. Pat. Nos. 4,027,821 and 4,044,754, freeze protection is provided by sensing temperature differential or temperature in the water in the collector. However, these flat plate collector systems provide for continuous operation during solar day periods in which water is pumped continuously through the collector and to the tank so long as solar radiation maintains a temperature differential value or the water in the collector is not approaching freezing point. By the present invention there is envisioned a need for more precise control over drainability and in particular to the operation of the advanced, evacuated tubular collectors, there is a need for control of the water circulation during operation to enhance performance.